The agency, which has not yet been named, will be led by MEC's global chief executive, Tim Castree.
Maxus global chief executive Lindsay Pattison will continue as Group M’s chief transformation officer.
Group M’s portfolio will now comprise three global media agency networks: Mindshare, MediaCom and the new company, plus digital agency Essence.
A spokesman for Group M said there were "surprisingly few" client conflicts between MEC and Maxus. "Where there are conflicts we are working through that on a case-by-case basis," he said.
He added that a name for the agency was forthcoming. "Right now, we’re starting a six-month process. When you have companies of this size and scale, it's a process," the spokesman added.
"Maxus and MEC share common values and ambitions. Both networks have a strong local market presence and entrepreneurial drive. Together, we believe we can create an exciting new media, content and technology agency which we look forward to introducing soon," Castree said.
Group M is also planning new investments across all of its agencies and [m]Platform data and technology capabilities.
"We’re committed to improving our service to clients. These moves will give us greater focus, help us innovate, and improve our speed of delivery," Kelly Clark, global chief executive of Group M, said.
Group M plans to expand Essence, which it acquired in 2015, both geographically, and by adding traditional media capabilities. Essence will also lead several key Group M client relationships as part of this restructure.
"Our mission is to make advertising more valuable to the world; with this infusion of talent, capabilities and markets, we can do this now on a bigger stage," Christian Juhl, chief executive of Essence, said.
These organisational changes will allow Group M to "more meaningfully invest" in each agency's future, Pattison said. "We’ve clearly signalled our ambition to transform, and we mean business," she added.